As far as I know, you can file form I-90 to renew your card even if your greencard is already expired, but only if you are currently within the US (and if you are, don't leave until you have your new card)

As far as I know you have to produce a valid GC at the interview for naturalization, so you should apply for renewal of your GC and then apply for citizenship

My sister has been out of the country since 1986 and would now like to return to the US, she is still in posession of her original green card and social security card, how do we find out if it's still valid and if not who do I contact to help her re-new it?

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">When your sister left America in order to live in another country, she relinquished her permanent residency status. </div></BLOCKQUOTE>

Not necessarily true. If she maintained ties to the US there is a chance she could return but it might be an uphill struggle given the absence for so long. She would need to file for an SB-1 visa.

"What you see in the photograph isn't what you saw at the time. The real skill of photography is organized visual lying."